1. Field of the Invention
The present invention relates generally to a charge coupled device (CCD). More particularly, the invention relates to a gate input system for a CCD which can switch the signal phase relationship between the input signal and output signal.
2. Description of the Background Art
The CCD has typically been used as signal delay element. Particularly, in the modern video signal processing technology, it has become necessary to use the CCD for a delay element in comb filters. Namely, in the prior art, the glass delay line has been used for forming a delay element in the comb filter. The glass delay element converts a signal into an acoustic wave for passage through a glass layer. Such a glass delay element, however, is limited by the reduction of the size and by the lowering of production cost. Furthermore, the glass delay element cannot successfully follow the trend in the video technology to use higher band. Due to such defects in the glass delay element, the CCD is considered as the replacement for the glass delay element.
In the video technology, the current trend is to make video equipments, such as a video camera, a video tape recorder and so forth, portable. Reduction in the video equipments size creates a need for operation at a lower power source voltage. In order to answer to this need, it becomes necessary to drive the CCD with a sole power source of relatively low voltage, e.g. 5 V. This leads to a difficulty in obtaining satisfactory high linearity in the input/output characteristics in a diode cut-off input system for applying the input signal to a source region of the CCD. For this reason, the needs for the gate input system increases.
In the gate input system, signal phases of the input signal and the output signal are necessarily reversed. For preventing this, an inverting amplifier is provided for the input or output circuit of the CCD. If the inverting amplifier is provided in the output circuit, it has to amplify the signal already amplified by the output amplifier. This may require wider dynamic range for the inverting amplifier. In view of this, it is preferred to provide the inverting amplifier in the input circuit.
When the inverting amplifier in the CCD provides the input and output signals in phase, the temperature characteristics of the CCD can be degraded. Particularly, the presence of the inverting amplifier can increase the temperature dependency of the delay magnitude for the signal. Fluctuations in the delay magnitude may affect characteristics of the comb filter. Because of this, two different requirements arise from customers. One is to permit degradation of the temperature characteristics in order to obtain coincident phases of input and output signals. The other is to prevent degradation of the temperature characteristics but permit difference in the signal phases of the input and output signals. It is not practical, however, to produce different types of CCD's in the viewpoint of manufacturing efficiency and cost. Therefore, it is preferred to produce a CCD which can answer both requirements.
On the other hand, the input bias for the delay line is variable depending upon the phase relationship between the input signal and the output signal. Therefore, it is preferred to provide the capability the switching of input bias level between different levels.